URL Rewriting Guide

Originally written byRalf S. Engelschall <rse@apache.org>
December 1997

This document supplements the mod_rewritereference documentation.
It describes how one can use Apache's mod_rewrite
to solve typical URL-based problems with which webmasters are
commonony confronted. We give detailed descriptions on how to
solve each problem by configuring URL rewriting rulesets.

The Apache module mod_rewrite is a killer
one, i.e. it is a really sophisticated module which provides
a powerful way to do URL manipulations. With it you can do nearly
all types of URL manipulations you ever dreamed about.
The price you have to pay is to accept complexity, because
mod_rewrite's major drawback is that it is
not easy to understand and use for the beginner. And even
Apache experts sometimes discover new aspects where
mod_rewrite can help.

In other words: With mod_rewrite you either
shoot yourself in the foot the first time and never use it again
or love it for the rest of your life because of its power.
This paper tries to give you a few initial success events to
avoid the first case by presenting already invented solutions
to you.

Here come a lot of practical solutions I've either invented
myself or collected from other people's solutions in the past.
Feel free to learn the black magic of URL rewriting from
these examples.

ATTENTION: Depending on your server-configuration
it can be necessary to slightly change the examples for your
situation, e.g. adding the [PT] flag when
additionally using mod_alias and
mod_userdir, etc. Or rewriting a ruleset
to fit in .htaccess context instead
of per-server context. Always try to understand what a
particular ruleset really does before you use it. It
avoid problems.

Canonical URLs

Description:

On some webservers there are more than one URL for a
resource. Usually there are canonical URLs (which should be
actually used and distributed) and those which are just
shortcuts, internal ones, etc. Independent of which URL the
user supplied with the request he should finally see the
canonical one only.

Solution:

We do an external HTTP redirect for all non-canonical
URLs to fix them in the location view of the Browser and
for all subsequent requests. In the example ruleset below
we replace /~user by the canonical
/u/user and fix a missing trailing slash for
/u/user.

Canonical Hostnames

Description:

The goal of this rule is to force the use of a particular
hostname, in preference to other hostnames which may be used to
reach the same site. For example, if you wish to force the use
of www.example.com instead of
example.com, you might use a variant of the
following recipe.

Moved DocumentRoot

Description:

Usually the DocumentRoot
of the webserver directly relates to the URL "/".
But often this data is not really of top-level priority, it is
perhaps just one entity of a lot of data pools. For instance at
our Intranet sites there are /e/www/
(the homepage for WWW), /e/sww/ (the homepage for
the Intranet) etc. Now because the data of the DocumentRoot stays at /e/www/ we had
to make sure that all inlined images and other stuff inside this
data pool work for subsequent requests.

Trailing Slash Problem

Description:

Every webmaster can sing a song about the problem of
the trailing slash on URLs referencing directories. If they
are missing, the server dumps an error, because if you say
/~quux/foo instead of /~quux/foo/
then the server searches for a file named
foo. And because this file is a directory it
complains. Actually it tries to fix it itself in most of
the cases, but sometimes this mechanism need to be emulated
by you. For instance after you have done a lot of
complicated URL rewritings to CGI scripts etc.

Solution:

The solution to this subtle problem is to let the server
add the trailing slash automatically. To do this
correctly we have to use an external redirect, so the
browser correctly requests subsequent images etc. If we
only did a internal rewrite, this would only work for the
directory page, but would go wrong when any images are
included into this page with relative URLs, because the
browser would request an in-lined object. For instance, a
request for image.gif in
/~quux/foo/index.html would become
/~quux/image.gif without the external
redirect!

So, to do this trick we write:

RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo$ foo/ [R]

The crazy and lazy can even do the following in the
top-level .htaccess file of their homedir.
But notice that this creates some processing
overhead.

Webcluster through Homogeneous URL Layout

Description:

We want to create a homogeneous and consistent URL
layout over all WWW servers on a Intranet webcluster, i.e.
all URLs (per definition server local and thus server
dependent!) become actually server independent!
What we want is to give the WWW namespace a consistent
server-independent layout: no URL should have to include
any physically correct target server. The cluster itself
should drive us automatically to the physical target
host.

Solution:

First, the knowledge of the target servers come from
(distributed) external maps which contain information
where our users, groups and entities stay. The have the
form

user1 server_of_user1
user2 server_of_user2
: :

We put them into files map.xxx-to-host.
Second we need to instruct all servers to redirect URLs
of the forms

when the URL is not locally valid to a server. The
following ruleset does this for us by the help of the map
files (assuming that server0 is a default server which
will be used if a user has no entry in the map):

Move Homedirs to Different Webserver

Description:

Many webmasters have asked for a solution to the
following situation: They wanted to redirect just all
homedirs on a webserver to another webserver. They usually
need such things when establishing a newer webserver which
will replace the old one over time.

Solution:

The solution is trivial with mod_rewrite.
On the old webserver we just redirect all
/~user/anypath URLs to
http://newserver/~user/anypath.

RewriteEngine on
RewriteRule ^/~(.+) http://newserver/~$1 [R,L]

Structured Homedirs

Description:

Some sites with thousands of users usually use a
structured homedir layout, i.e. each homedir is in a
subdirectory which begins for instance with the first
character of the username. So, /~foo/anypath
is /home/f/foo/.www/anypath
while /~bar/anypath is
/home/b/bar/.www/anypath.

Solution:

We use the following ruleset to expand the tilde URLs
into exactly the above layout.

Filesystem Reorganization

Description:

This really is a hardcore example: a killer application
which heavily uses per-directory
RewriteRules to get a smooth look and feel
on the Web while its data structure is never touched or
adjusted. Background: net.sw is
my archive of freely available Unix software packages,
which I started to collect in 1992. It is both my hobby
and job to to this, because while I'm studying computer
science I have also worked for many years as a system and
network administrator in my spare time. Every week I need
some sort of software so I created a deep hierarchy of
directories where I stored the packages:

In July 1996 I decided to make this archive public to
the world via a nice Web interface. "Nice" means that I
wanted to offer an interface where you can browse
directly through the archive hierarchy. And "nice" means
that I didn't wanted to change anything inside this
hierarchy - not even by putting some CGI scripts at the
top of it. Why? Because the above structure should be
later accessible via FTP as well, and I didn't want any
Web or CGI stuff to be there.

Solution:

The solution has two parts: The first is a set of CGI
scripts which create all the pages at all directory
levels on-the-fly. I put them under
/e/netsw/.www/ as follows:

The DATA/ subdirectory holds the above
directory structure, i.e. the real
net.sw stuff and gets
automatically updated via rdist from time to
time. The second part of the problem remains: how to link
these two structures together into one smooth-looking URL
tree? We want to hide the DATA/ directory
from the user while running the appropriate CGI scripts
for the various URLs. Here is the solution: first I put
the following into the per-directory configuration file
in the DocumentRoot
of the server to rewrite the announced URL
/net.sw/ to the internal path
/e/netsw:

RewriteRule ^net.sw$ net.sw/ [R]
RewriteRule ^net.sw/(.*)$ e/netsw/$1

The first rule is for requests which miss the trailing
slash! The second rule does the real thing. And then
comes the killer configuration which stays in the
per-directory config file
/e/netsw/.www/.wwwacl:

Notice the L (last) flag and no
substitution field ('-') in the forth part

Notice the ! (not) character and
the C (chain) flag at the first rule
in the last part

Notice the catch-all pattern in the last rule

NCSA imagemap to Apache mod_imap

Description:

When switching from the NCSA webserver to the more
modern Apache webserver a lot of people want a smooth
transition. So they want pages which use their old NCSA
imagemap program to work under Apache with the
modern mod_imap. The problem is that there
are a lot of hyperlinks around which reference the
imagemap program via
/cgi-bin/imagemap/path/to/page.map. Under
Apache this has to read just
/path/to/page.map.

Solution:

We use a global rule to remove the prefix on-the-fly for
all requests:

RewriteEngine on
RewriteRule ^/cgi-bin/imagemap(.*) $1 [PT]

Search pages in more than one directory

Description:

Sometimes it is necessary to let the webserver search
for pages in more than one directory. Here MultiViews or
other techniques cannot help.

Solution:

We program a explicit ruleset which searches for the
files in the directories.

RewriteEngine on
# first try to find it in custom/...
# ...and if found stop and be happy:
RewriteCond /your/docroot/dir1/%{REQUEST_FILENAME} -f
RewriteRule ^(.+) /your/docroot/dir1/$1 [L]
# second try to find it in pub/...
# ...and if found stop and be happy:
RewriteCond /your/docroot/dir2/%{REQUEST_FILENAME} -f
RewriteRule ^(.+) /your/docroot/dir2/$1 [L]
# else go on for other Alias or ScriptAlias directives,
# etc.
RewriteRule ^(.+) - [PT]

Set Environment Variables According To URL Parts

Description:

Perhaps you want to keep status information between
requests and use the URL to encode it. But you don't want
to use a CGI wrapper for all pages just to strip out this
information.

Solution:

We use a rewrite rule to strip out the status information
and remember it via an environment variable which can be
later dereferenced from within XSSI or CGI. This way a
URL /foo/S=java/bar/ gets translated to
/foo/bar/ and the environment variable named
STATUS is set to the value "java".

RewriteEngine on
RewriteRule ^(.*)/S=([^/]+)/(.*) $1/$3 [E=STATUS:$2]

Virtual User Hosts

Description:

Assume that you want to provide
www.username.host.domain.com
for the homepage of username via just DNS A records to the
same machine and without any virtualhosts on this
machine.

Solution:

For HTTP/1.0 requests there is no solution, but for
HTTP/1.1 requests which contain a Host: HTTP header we
can use the following ruleset to rewrite
http://www.username.host.com/anypath
internally to /home/username/anypath:

Redirect Failing URLs To Other Webserver

Description:

A typical FAQ about URL rewriting is how to redirect
failing requests on webserver A to webserver B. Usually
this is done via ErrorDocument CGI-scripts in Perl, but
there is also a mod_rewrite solution.
But notice that this performs more poorly than using an
ErrorDocument
CGI-script!

Solution:

The first solution has the best performance but less
flexibility, and is less error safe:

This uses the URL look-ahead feature of mod_rewrite.
The result is that this will work for all types of URLs
and is a safe way. But it does a performance impact on
the webserver, because for every request there is one
more internal subrequest. So, if your webserver runs on a
powerful CPU, use this one. If it is a slow machine, use
the first approach or better a ErrorDocument CGI-script.

Extended Redirection

Description:

Sometimes we need more control (concerning the
character escaping mechanism) of URLs on redirects.
Usually the Apache kernels URL escape function also
escapes anchors, i.e. URLs like "url#anchor".
You cannot use this directly on redirects with
mod_rewrite because the
uri_escape() function of Apache
would also escape the hash character.
How can we redirect to such a URL?

Solution:

We have to use a kludge by the use of a NPH-CGI script
which does the redirect itself. Because here no escaping
is done (NPH=non-parseable headers). First we introduce a
new URL scheme xredirect: by the following
per-server config-line (should be one of the last rewrite
rules):

This provides you with the functionality to do
redirects to all URL schemes, i.e. including the one
which are not directly accepted by mod_rewrite.
For instance you can now also redirect to
news:newsgroup via

RewriteRule ^anyurl xredirect:news:newsgroup

Notice: You have not to put [R] or
[R,L] to the above rule because the
xredirect: need to be expanded later
by our special "pipe through" rule above.

Archive Access Multiplexer

Description:

Do you know the great CPAN (Comprehensive Perl Archive
Network) under http://www.perl.com/CPAN?
This does a redirect to one of several FTP servers around
the world which carry a CPAN mirror and is approximately
near the location of the requesting client. Actually this
can be called an FTP access multiplexing service. While
CPAN runs via CGI scripts, how can a similar approach
implemented via mod_rewrite?

Solution:

First we notice that from version 3.0.0
mod_rewrite can
also use the "ftp:" scheme on redirects.
And second, the location approximation can be done by a
RewriteMap
over the top-level domain of the client.
With a tricky chained ruleset we can use this top-level
domain as a key to our multiplexing map.

From Old to New (intern)

Description:

Assume we have recently renamed the page
foo.html to bar.html and now want
to provide the old URL for backward compatibility. Actually
we want that users of the old URL even not recognize that
the pages was renamed.

Solution:

We rewrite the old URL to the new one internally via the
following rule:

RewriteEngine on
RewriteBase /~quux/
RewriteRule ^foo\.html$ bar.html

From Old to New (extern)

Description:

Assume again that we have recently renamed the page
foo.html to bar.html and now want
to provide the old URL for backward compatibility. But this
time we want that the users of the old URL get hinted to
the new one, i.e. their browsers Location field should
change, too.

Solution:

We force a HTTP redirect to the new URL which leads to a
change of the browsers and thus the users view:

Browser Dependent Content

Description:

At least for important top-level pages it is sometimes
necessary to provide the optimum of browser dependent
content, i.e. one has to provide a maximum version for the
latest Netscape variants, a minimum version for the Lynx
browsers and a average feature version for all others.

Solution:

We cannot use content negotiation because the browsers do
not provide their type in that form. Instead we have to
act on the HTTP header "User-Agent". The following condig
does the following: If the HTTP header "User-Agent"
begins with "Mozilla/3", the page foo.html
is rewritten to foo.NS.html and and the
rewriting stops. If the browser is "Lynx" or "Mozilla" of
version 1 or 2 the URL becomes foo.20.html.
All other browsers receive page foo.32.html.
This is done by the following ruleset:

Dynamic Mirror

Description:

Assume there are nice webpages on remote hosts we want
to bring into our namespace. For FTP servers we would use
the mirror program which actually maintains an
explicit up-to-date copy of the remote data on the local
machine. For a webserver we could use the program
webcopy which acts similar via HTTP. But both
techniques have one major drawback: The local copy is
always just as up-to-date as often we run the program. It
would be much better if the mirror is not a static one we
have to establish explicitly. Instead we want a dynamic
mirror with data which gets updated automatically when
there is need (updated data on the remote host).

Solution:

To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the Proxy Throughput feature
(flag [P]):

Reverse Dynamic Mirror

Retrieve Missing Data from Intranet

Description:

This is a tricky way of virtually running a corporate
(external) Internet webserver
(www.quux-corp.dom), while actually keeping
and maintaining its data on a (internal) Intranet webserver
(www2.quux-corp.dom) which is protected by a
firewall. The trick is that on the external webserver we
retrieve the requested data on-the-fly from the internal
one.

Solution:

First, we have to make sure that our firewall still
protects the internal webserver and that only the
external webserver is allowed to retrieve data from it.
For a packet-filtering firewall we could for instance
configure a firewall ruleset like the following:

Load Balancing

Suppose we want to load balance the traffic to
www.foo.com over www[0-5].foo.com
(a total of 6 servers). How can this be done?

Solution:

There are a lot of possible solutions for this problem.
We will discuss first a commonly known DNS-based variant
and then the special one with mod_rewrite:

DNS Round-Robin

The simplest method for load-balancing is to use
the DNS round-robin feature of BIND.
Here you just configure www[0-9].foo.com
as usual in your DNS with A(address) records, e.g.

www0 IN A 1.2.3.1
www1 IN A 1.2.3.2
www2 IN A 1.2.3.3
www3 IN A 1.2.3.4
www4 IN A 1.2.3.5
www5 IN A 1.2.3.6

Then you additionally add the following entry:

www IN CNAME www0.foo.com.
IN CNAME www1.foo.com.
IN CNAME www2.foo.com.
IN CNAME www3.foo.com.
IN CNAME www4.foo.com.
IN CNAME www5.foo.com.
IN CNAME www6.foo.com.

Notice that this seems wrong, but is actually an
intended feature of BIND and can be used
in this way. However, now when www.foo.com gets
resolved, BIND gives out www0-www6
- but in a slightly permutated/rotated order every time.
This way the clients are spread over the various
servers. But notice that this not a perfect load
balancing scheme, because DNS resolve information
gets cached by the other nameservers on the net, so
once a client has resolved www.foo.com
to a particular wwwN.foo.com, all
subsequent requests also go to this particular name
wwwN.foo.com. But the final result is
ok, because the total sum of the requests are really
spread over the various webservers.

In this variant we use mod_rewrite
and its proxy throughput feature. First we dedicate
www0.foo.com to be actually
www.foo.com by using a single

www IN CNAME www0.foo.com.

entry in the DNS. Then we convert
www0.foo.com to a proxy-only server,
i.e. we configure this machine so all arriving URLs
are just pushed through the internal proxy to one of
the 5 other servers (www1-www5). To
accomplish this we first establish a ruleset which
contacts a load balancing script lb.pl
for all URLs.

A last notice: Why is this useful? Seems like
www0.foo.com still is overloaded? The
answer is yes, it is overloaded, but with plain proxy
throughput requests, only! All SSI, CGI, ePerl, etc.
processing is completely done on the other machines.
This is the essential point.

Hardware/TCP Round-Robin

There is a hardware solution available, too. Cisco
has a beast called LocalDirector which does a load
balancing at the TCP/IP level. Actually this is some
sort of a circuit level gateway in front of a
webcluster. If you have enough money and really need
a solution with high performance, use this one.

New MIME-type, New Service

Description:

On the net there are a lot of nifty CGI programs. But
their usage is usually boring, so a lot of webmaster
don't use them. Even Apache's Action handler feature for
MIME-types is only appropriate when the CGI programs
don't need special URLs (actually PATH_INFO
and QUERY_STRINGS) as their input. First,
let us configure a new file type with extension
.scgi (for secure CGI) which will be processed
by the popular cgiwrap program. The problem
here is that for instance we use a Homogeneous URL Layout
(see above) a file inside the user homedirs has the URL
/u/user/foo/bar.scgi. But
cgiwrap needs the URL in the form
/~user/foo/bar.scgi/. The following rule
solves the problem:

Or assume we have some more nifty programs:
wwwlog (which displays the
access.log for a URL subtree and
wwwidx (which runs Glimpse on a URL
subtree). We have to provide the URL area to these
programs so they know on which area they have to act on.
But usually this ugly, because they are all the times
still requested from that areas, i.e. typically we would
run the swwidx program from within
/u/user/foo/ via hyperlink to

/internal/cgi/user/swwidx?i=/u/user/foo/

which is ugly. Because we have to hard-code
both the location of the area
and the location of the CGI inside the
hyperlink. When we have to reorganize the area, we spend a
lot of time changing the various hyperlinks.

Solution:

The solution here is to provide a special new URL format
which automatically leads to the proper CGI invocation.
We configure the following:

The same approach leads to an invocation for the
access log CGI program when the hyperlink
:log gets used.

From Static to Dynamic

Description:

How can we transform a static page
foo.html into a dynamic variant
foo.cgi in a seamless way, i.e. without notice
by the browser/user.

Solution:

We just rewrite the URL to the CGI-script and force the
correct MIME-type so it gets really run as a CGI-script.
This way a request to /~quux/foo.html
internally leads to the invocation of
/~quux/foo.cgi.

On-the-fly Content-Regeneration

Description:

Here comes a really esoteric feature: Dynamically
generated but statically served pages, i.e. pages should be
delivered as pure static pages (read from the filesystem
and just passed through), but they have to be generated
dynamically by the webserver if missing. This way you can
have CGI-generated pages which are statically served unless
one (or a cronjob) removes the static contents. Then the
contents gets refreshed.

Here a request to page.html leads to a
internal run of a corresponding page.cgi if
page.html is still missing or has filesize
null. The trick here is that page.cgi is a
usual CGI script which (additionally to its STDOUT)
writes its output to the file page.html.
Once it was run, the server sends out the data of
page.html. When the webmaster wants to force
a refresh the contents, he just removes
page.html (usually done by a cronjob).

Document With Autorefresh

Description:

Wouldn't it be nice while creating a complex webpage if
the webbrowser would automatically refresh the page every
time we write a new version from within our editor?
Impossible?

Solution:

No! We just combine the MIME multipart feature, the
webserver NPH feature and the URL manipulation power of
mod_rewrite. First, we establish a new
URL feature: Adding just :refresh to any
URL causes this to be refreshed every time it gets
updated on the filesystem.

Mass Virtual Hosting

Description:

The <VirtualHost> feature of Apache is nice
and works great when you just have a few dozens
virtual hosts. But when you are an ISP and have hundreds of
virtual hosts to provide this feature is not the best
choice.

Solution:

To provide this feature we map the remote webpage or even
the complete remote webarea to our namespace by the use
of the Proxy Throughput feature (flag [P]):

Blocking of Robots

Description:

How can we block a really annoying robot from
retrieving pages of a specific webarea? A
/robots.txt file containing entries of the
"Robot Exclusion Protocol" is typically not enough to get
rid of such a robot.

Solution:

We use a ruleset which forbids the URLs of the webarea
/~quux/foo/arc/ (perhaps a very deep
directory indexed area where the robot traversal would
create big server load). We have to make sure that we
forbid access only to the particular robot, i.e. just
forbidding the host where the robot runs is not enough.
This would block users from this host, too. We accomplish
this by also matching the User-Agent HTTP header
information.

Blocked Inline-Images

Description:

Assume we have under http://www.quux-corp.de/~quux/
some pages with inlined GIF graphics. These graphics are
nice, so others directly incorporate them via hyperlinks to
their pages. We don't like this practice because it adds
useless traffic to our server.

Solution:

While we cannot 100% protect the images from inclusion,
we can at least restrict the cases where the browser
sends a HTTP Referer header.

##
## hosts.deny
##
## ATTENTION! This is a map, not a list, even when we treat it as such.
## mod_rewrite parses it for key/value pairs, so at least a
## dummy value "-" must be present for each entry.
##
193.102.180.41 -
bsdti1.sdm.de -
192.76.162.40 -

Proxy Deny

Description:

How can we forbid a certain host or even a user of a
special host from using the Apache proxy?

Solution:

We first have to make sure mod_rewrite
is below(!) mod_proxy in the Configuration
file when compiling the Apache webserver. This way it gets
called beforemod_proxy. Then we
configure the following for a host-dependent deny...

Special Authentication Variant

Description:

Sometimes a very special authentication is needed, for
instance a authentication which checks for a set of
explicitly configured users. Only these should receive
access and without explicit prompting (which would occur
when using the Basic Auth via mod_auth).

Solution:

We use a list of rewrite conditions to exclude all except
our friends:

External Rewriting Engine

Description:

A FAQ: How can we solve the FOO/BAR/QUUX/etc.
problem? There seems no solution by the use of
mod_rewrite...

Solution:

Use an external RewriteMap, i.e. a program which acts
like a RewriteMap. It is run once on startup of Apache
receives the requested URLs on STDIN and has
to put the resulting (usually rewritten) URL on
STDOUT (same order!).

#!/path/to/perl
# disable buffered I/O which would lead
# to deadloops for the Apache server
$| = 1;
# read URLs one per line from stdin and
# generate substitution URL on stdout
while (<>) {
s|^foo/|bar/|;
print $_;
}

This is a demonstration-only example and just rewrites
all URLs /~quux/foo/... to
/~quux/bar/.... Actually you can program
whatever you like. But notice that while such maps can be
used also by an average user, only the
system administrator can define it.